1. Field of the Invention
The present invention relates to an image reading apparatus for reading a transported original document, and more specifically, to an image reading apparatus having a shading unit provided opposite to a reading surface to read an original document.
2. Description of the Related Art
There has conventionally been known the so-called sheet-through type image reading apparatus which is an apparatus for reading an image of an original document passed through a reading section of a transport path by a reading unit at rest. In such an image reading apparatus, concentration differences occur in a read image by fluctuations in sensitivity for each pixel of a photoelectric converter constituting an image reading means, fluctuations in quantity of light and characteristics of a light source, reflecting mirrors and the like, variations in image formation by a lens and the like, etc. To correct the concentration differences, a shading plate formed of a plate-shaped white member is disposed in a position opposite to a reading platen, the shading plate is read at arbitrary timing to obtain shading correction data, and an excellent read image is formed by capturing the shading correction data into the image data (for example, Japanese Laid-Open Patent Publication Nos. H05-130330 and H07-231378).
In general, the shading plate often has a structure where the plate is fixed in the position opposite to the reading surface on the reading unit side, or the plate is attached by inserting into an insertion hole in the opposite position provided integrally with the reading surface.
However, in the structure where the white shading plate to acquire the shading correction data is fixed in the position opposite to the reading surface, when dust, stain or the like adheres to the surface of the shading plate, to remove the dust or stain, the problem arises that the apparatus body with the shading plate attached needs to be disassembled. Further, when the shading plate is attached after once the apparatus body is disassembled, another problem occurs that alignment with the reading surface is not easy.
Moreover, even when part of the shading plate is stained, the shading plate needs to be replaced. Alternately, when the shading plate is in film form, a new film needs to be bonded after removing the old film. Therefore, it takes time to perform cleaning and maintenance, interfering with the operation of the image reading apparatus.
Hence, it is an object of the invention to provide a document reading apparatus having a detachable structure to facilitate cleaning and maintenance of a shading plate to form a transport path of an original document and perform shading processing, while enabling reading processing for an excellent image to be executed even when the shading plate is stained or the like.